Jamieson, Fausset and Brown on Nahum 2:8

8. But--rather, "Though" [G. V. Smith].

of old--rather, "from the days that she hath been"; from the earliest period of her existence. Alluding to Nineveh's antiquity (Ge 10:11). "Though Nineveh has been of old defended by water surrounding her, yet her inhabitants shall flee away." Grotius, less probably (compare Na 3:8-12), interprets, the "waters" of her numerous population (Is 8:7; Jr 51:13; Re 17:15).

Stand, stand, shall they cry--that is, the few patriotic citizens shall cry to their fleeing countrymen; "but none looketh back," much less stops in flight, so panic-stricken are they.